The present invention relates to a measuring method for an ellipsometric parameter, which is designed to measure ellipsometric parameters used to measure the thickness of a thin film, and an ellipsometer for measuring ellipsometric parameters by using this measuring method.
As a method of measuring the thickness of a thin film, ellipsometry is used. In this method, a change in polarization state upon reflection of a beam by a sample surface, i.e., a ratio .rho. between a reflectance Rp of a light component (P component) parallel to the incident plane of an electric field vector and a reflectance Rs of a light component (S component) perpendicular thereto, is measured according to equation (1), and a film thickness d is obtained in accordance with a predetermined relationship between the already obtained polarization reflectance ratio .rho. and the film thickness d: EQU .rho.=Rp/Rs=tan.PSI.exp[j.DELTA.] (1)
In this case, since the polarization reflectance ratio .rho. is normally represented by a complex number as indicated by equation (1), two ellipsometric parameters, i.e., an amplitude ratio .PSI. and a phase difference .DELTA., must be obtained.
As a conventional method of obtaining these ellipsometric parameters .PSI. and .DELTA., the null ellipsometry method is known. In this method, a polarized beam is radiated from a light source onto a measurement target at a predetermined angle with respect to the target, and a beam reflected by the target, which is elliptically polarized, is transmitted through a .lambda./4 plate and an analyzer to be guided to a light-receiving unit. While optical intensity signals obtained by the light-receiving unit are observed through, e.g., a measuring unit, the .lambda./4 plate and the analyzer are rotated to obtain a rotational angle at which the minimum optical intensity is observed. The above-mentioned ellipsometric parameters are calculated on the basis of this rotational angle. In this method, however, since the analyzer needs to be rotated to find an angular position at which the minimum optical intensity is observed, a measuring operation requires a long period of time.
Under the circumstances, a method using a rotating analyzer is proposed as a method of measuring the above ellipsometric parameters at a relatively high speed. More specifically, according to this method, similar to the above-described null ellipsometry, a linearly polarized beam with predetermined azimuth angle, e.g. 45.degree., is radiated from a light source onto a measurement target, at a predetermined angle with respect to the target, and the ellipsometric parameters are calculated on the basis of the output waveforms of optical intensity signals obtained by a light-receiving unit when the analyzer on the measurement side is rotated once.
In the measuring method for an ellipsometric parameter, which uses the rotating analyzer, however, the following problems are posed.
(1) The analyzer must be rotated once to execute one measuring operation. This rotation requires a predetermined period of time or more. Therefore, it is impossible to obtain the ellipsometric parameters .PSI. and .DELTA. of a measurement target which is moving at high speed and to measure the film thickness d on the basis of these ellipsometric parameters. In addition, the presence of a mechanical movable portion increases the size of the apparatus itself. For this reason, the apparatus cannot be installed on a production line in a factory to perform on-line measurement of film thicknesses on measurement targets, e.g., continuously supplied measurement targets.
(2) If, for example, the above-mentioned linearly polarized beam is used as a beam to be incident on a measurement target, phase difference information of ellipsometric parameters is obtained in the form of cos.DELTA.. Therefore, it is impossible to determine whether a correct phase difference is .DELTA. or (360.degree.-.DELTA.). In addition, if a circularly polarized beam is used as an incident beam, since phase difference information of ellipsometric parameters is obtained in the form of sin.DELTA., it is impossible to determine whether a correct phase difference is .DELTA. or (90.degree.-.DELTA.).
It is, therefore, necessary to determine which quadrant (zone) of the first quadrant (0.degree. to 90.degree. ) to the fourth quadrant (270.degree. to 360.degree. ) the obtained phase difference .DELTA. belongs to. This determination is generally called zone definition. In a general zone definition method, output waveforms are respectively obtained from a circularly polarized incident beam, which is formed by inserting a .lambda./4 plate in the optical path of a beam to be incident on a measurement target, and from a linearly polarized incident beam, which is formed without inserting a .lambda./4 plate in the optical path, and the two output waveforms are compared with each other to determine a zone to which the obtained phase difference belongs. However, in this method, since two measuring operations must be performed with respect to the same measurement point, the measurement efficiency is not necessarily increased much as compared with the above-described null ellipsometry.
(3) Although a beam reflected by a measurement target is elliptically polarized, the measurement precision is decreased as the elliptic shape approaches a circle. For this reason, a .lambda./4 plate needs to be inserted in the optical path of an incident or reflected beam to change the elliptical polarization state of a reflected beam to be incident on a measurement system. This further decreases the measurement efficiency.
(4) The conventional method requires a mechanism for rotating the analyzer and a mechanism for inserting/removing a .lambda./4 plate in/from an optical path. Since these mechanisms incorporate a large number of movable members, the apparatus itself is increased in size. Therefore, a place where the ellipsometer is installed is limited to a relatively large area with a good environment, e.g., a laboratory.
In addition, since the above-mentioned members are mechanically moved, a long period of time is required for moving operations. If the .lambda./4 plate is inserted/removed and the analyzer is rotated twice with respect to one measurement point, it takes a few seconds to perform a measurement with respect to one measurement point. Therefore, high-speed measurement in an on-line state cannot be performed.
The present invention has been made in consideration of the above situation, and has as its object to provide a measuring method for ellipsometric parameters and an ellipsometer, in which a reflected beam having an elliptically polarized beam, which is reflected by a measurement target, is divided into four different polarized light components, and the optical intensities of the respective polarized light components are detected to calculate ellipsometric parameters on the basis of the detected four optical intensities, so that the analyzer need not be rotated, the .lambda./4 plate need not be inserted/removed in/from an optical path, and only stationary members are required, thereby saving the time required to move movable members, allowing substantially instant measurement of ellipsometric parameters by one measuring operation, enabling on-line measurement of ellipsometric parameters with respect to moving measurement targets, and allowing measurement of film thicknesses.